1. Field of the Invention
The present invention relates to an impact printhead in which a plurality of impact wires are driven to print characters on a medium.
2. Description of the Related Art
A dot matrix printer or impact matrix printer employs a printhead that runs back and forth on the page and prints by impact, striking an ink-soaked cloth ribbon against the paper. Impact printers are capable of printing on a variety of media at low cost, and find their application in a variety of areas including an output device in an information processing system. Impact printheads may be of variety of types including a plunger type, a spring-charge type, and a clapper type.
For an impact type printhead, impact wires are fixed to ends of armatures supported on one end of a cantilever type flat spring. A permanent magnet attracts the armature to a core such that mechanical energy is stored in the spring. When printing is performed, current is applied to a coil wound around the core, thereby producing a magnetic flux in a direction opposite to the magnetic flux of the permanent magnet. The magnetic force produced by the produced magnetic flux overcomes the magnetic force produced by the permanent magnet, allowing the flat spring to drive the impact wire. The impact wire moves toward its free end to its extended position, thereby striking the ink ribbon against the print medium to print dots.
FIG. 19 illustrates a conventional impact printhead 1. Referring to FIG. 19, an impact printhead 1 includes a plurality of impact wires 2. The impact wires 2 are each secured at one end thereof to the rip of an armature 3. The armature 3 is fixed to a flat spring 4, so that the armature 3 and flat spring 4 move in unison. The base of the flat spring 4 is, for example, welded to a spacer 5 of a stacked structure of the spacer 5, a yoke 6, a permanent magnet 7, and a yoke 8.
A core 10 is mounted to a base yoke 9 located at the base portion of the impact printhead 1. A coil 11 is wound around the core 10. Current is applied to the coil 11 under control of a controller 12. The core 10 and the flat spring 4 are positioned relative to each other with a gap Δg between them. The impact wires 2 extend through a vibration restricting guide 14, and further extend through a wire guide 13 such that the free end portions of the impact wires 2 extend through the wire guide 13.
FIG. 20 illustrates the impact printhead 1 when it is not operating. The armature 3 remains attracted by the flux of the permanent magnet 7 to the core 10. The flat spring 4 flexes to the core so that mechanical energy is stored in the flat spring 4. At this moment, the impact wire 2 is restricted in its movement both in an X direction and in a Y direction by the wire guide 13. In other words, when the impact printhead 1 is not driven, the middle portion of the impact wire 2 has flexed by Δy in the Y direction from its position when the impact printhead 1 is driven.
When the controller 12 applies voltage across the coil 11, the coil 11 produces a magnetic flux in a direction opposite to the magnetic flux of the permanent magnet 7, the magnetic flux produced by the coil 11 overcoming the magnetic flux of the permanent magnet 7. Thus, the impact wire 2 projects outwardly from the wire guide 13 in a Z direction to strike the ink soaked cloth ribbon against the print medium. Subsequently, when the controller 12 shuts off the voltage across the coil 11, the core 10 again attracts the armature 3 so that the armature 3 remains attracted until the controller 12 applies voltage across the coil 11 again.
When the impact wire 2 is driven, it moves to an extended position where the impact wire 2 strikes the ink-soaked cloth ribbon against the medium. When the impact wire 2 is not driven, it moves to a retracted position where the impact wire 2 does not strike the ink-soaked cloth ribbon against the medium. Because the impact wire 2 is an elastic body, it vibrates when it returns from the extended position to the retracted position. Thus, shortly after the impact wire 2 reaches the retracted position, the impact wire 2 continues to vibrate with the amplitude decreasing until it is completely damped (higher-order vibration mode). The connection between the impact wire 2 and the armature 3 may be damaged due to repetitive vibration of the impact wire 2. In order to prevent higher-order vibration of the impact wire 2, a vibration restricting guide 14 is provided.
FIG. 21 is a perspective view of the vibration restricting guide 14. FIG. 22 illustrates the operation of the vibration restricting guide 14. Referring to FIGS. 21 and 22, a plurality of holes 15 having a circular cross section are formed in the vibration restricting guide 14. The holes 15 have a diameter slightly larger than that of the impact wire 2. Referring to FIG. 22, if the vibration restricting guide 14 is not employed, the impact wire 2 would be at rest in a dotted line position in FIG. 22. If the vibration restricting guide 14 is employed, the wall 15a of the hole 15 holds the impact wire 2 in a solid line position in FIG. 22 where the wall 15a pushes the impact wire 2 to displace by ΔP from the dotted line position. Another way of looking at this situation is that the impact wire 2 resiliently pushes the wall 15a of the hole 15 with a pressing force of ΔPw. The pressing force ΔPw prevents higher-order vibration of the impact wire 2 after the impact wire 2 is in the attracted position.
With the aforementioned conventional apparatus equipped with a vibration restricting guide, the impact wire 2 is pressed against the wall 15a of the hole 15 at all times when the impact wire 2 is in its retracted state. Thus, repetitive impact operation of the impact wire 2 causes the impact wire 2 to repetitively beat the wall 15a, resulting in wear of the wall 15a. Wear of the wall 15a leads to a larger hole 15, decreasing the pressing force of the impact wire 2 exerted on the wall 15a. The wear becomes less effective in damping vibration, and the impact wire 2 and armature become apart eventually.
For prolonging the lifetime of the impact printhead, the vibration restricting guide should be formed of a highly wear-resistant material such as ceramics. Highly wear-resistant materials are more expensive than general purpose resin materials. Thus, the prolonged life and low cost of an impact printhead are difficult to achieve.